OS X: The problem with the Command + Q key command is that it's awfully close to lots of other key commands and easy to hit on accident. CommandQ is an OS X app that addresses that by making you hold the titular keystroke for two seconds before your open application actually closes.

It's a lot like a similar feature Chrome introduced some time ago, but for whatever apps you choose to enable it for via the CommandQ settings. At $3.99, it's a bit more expensive than what we'd like, but it does have a 30-day free trial available so you can see how it fits into your workflow.